To Make Chocolate Marshmallow Icing: In a saucepan over medium heat cook the 2-1/2 cups white sugar and the evaporated milk to the soft ball stage 238 degrees F (114 degrees C). Stir in semi-sweet chocolate chips, 1/2 cup butter or margarine and the marshmallow cream. Stir until all are melted. Remove from heat and beat until cooled. Spoon over top of cooled cake.